With conventional steering of boats with controllable propeller drives, a mechanical power transmission or mechanical power transmission connected to a hydraulic system is used for power amplification from a wheel to the propeller drive, an example of such a system being given in U.S. Pat. No. 5,399,112. This type of steering is well-suited for boats equipped with one drive, and for boats where the distance between the wheel and actuator for the controllable propeller drive is not such that the laying of cables between the wheel and actuator constitutes a problem.
For boats equipped with several drives and for boats where it is not desirable to have mechanical or hydraulic power transmission from the position where the wheel is located to actuators for setting the position of the propeller drives, it is expedient to utilize electronic control of the actuators. This applies in particular for a type of boat which is driven at planing speeds and is designed with a V-bottomed hull designed for planing, with an individually-controllable drive suspended on each side of the center line of the hull. These drives comprise an underwater housing projecting downwards from the outside of the hull, suspended in such way that it can be rotated in relation to the hull. A drive shaft is mounted in the underwater housing in such a way that it can rotate. The drive shaft drives a propeller shaft that is at least essentially horizontal via a bevel gear mechanism contained in the underwater housing. Such a type of boat is known in, for example, SE-9402272-0. As the drives are suspended at right angles to the bottom of the hull on each side of the center line of the V-shaped hull, the drive shafts will be angled in relation to each other. This means that a mechanical power transmission for steering both drives would be very complex, in particular in the case when individual steering of the drives is required in response to movements of the wheel.
To achieve the abovementioned object, it is advantageous to utilize electronic control of steering for a propeller drive on a boat comprising a propeller drive suspended in a housing that can be rotated.
With the utilization of electronic control systems for boats, it is of the greatest importance that the control systems should be reliable. Incorrect steering can result in unnecessary wear and tear on bearings and other components comprised in the boat's driveline. Incorrect steering can also mean that the boat's maximum performance cannot be utilized, which is the case when a boat equipped with two propeller drives does not correctly set the direction of the propeller drives and hence the direction of the propulsive thrust.
In order to ensure that the steering is correct, it is proposed in US 2003/0079668 that an electronic control system is continually calibrated. This patent application describes an electronic control system for a boat with waterjet operation. Calibration in association with starting up the vessel, so-called “dockside calibration”, is carried out, where all actuators for active steering of the waterjet unit's intake are moved from one end position to the other. At the same time, the helmsman is to move control devices in the form of a wheel and joystick to the respective extreme positions. Although this type of calibration ensures that the control system is functioning correctly, the calibration is time-consuming and also requires the helmsman's active participation. This means that the helmsman may perceive the calibration as troublesome and as a result may skip the calibration procedure. As the calibration, and hence the function test of the control system, requires something to be carried out by hand, there is also a danger that the helmsman will forget to carry out the calibration.
In the event that a fault arises in the control system as described in US 2003/0079668 that is of such a kind that the calibration function is not reliable, a boat that utilizes the system proposed therein will display unstable steering characteristics. By unstable steering characteristics is meant an unforeseeable deviation between the course specified by a control device and the course on which the boat is traveling.